THE VISIBLE STARS. 



663 



dark edge of the moon's disk, and then it is instantly extinguished, without the 

 slightest appearance of diminution of its brightness. This effect also presents 

 a striking proof of the non-existence of an atmosphere round the moon. 



It may be asked then, if such be the case, if none of the stars, great or small, 

 have any discoverable magnitude at all ; with what meaning can we speak of 

 stars of the first, second, or other orders of magnitude ? The term magnitude 

 thus applied, was used before the invention of the telescope, when the start, 

 j having been observed only with the naked eye, were really supposed to hare 

 different magnitudes. We must accept the term now used to express not 

 the comparative magnitude, but the comparative brightness of the stars. 

 Thus a star of the first magnitude, means of the greatest apparent brightness ; 

 a star of the second magnitude means that which is in the next degree of 

 splendor, and so on. But what are we to infer from this singular fact, that no 

 magnifying power, however great, will exhibit to us a star with any sensible 

 magnitude ? must we admit that the optical instrument loses its magnifying 

 power when applied to the stars, while it retains it with every other visible 

 object? Such a consequence would be eminently absurd. We are therefore 

 driven to an inference regarding the magnitude of stars as astonishing and al- 

 most as incredible as that which was forced upon us respecting their distan- 

 ces. We saw that the entire magnitude of the annual orbit of the earth, stupen- 

 dous as it is, was nothing compared to the distance of one of those bodies, and 

 consequently if that orbit were filled by a sun whose magnitude would there- 

 fore be infinitely greater than that of ours, such a sun would not appear to an 

 observer at the nearest star of greater magnitude than one third of a second ; 

 consequently would have no magnitude sensible to the eye, and would appear 

 as a mere lucid point to an observer at the star ! We are then prepared for 

 the inference respecting the fixed stars which the telescopic observations al- 

 ready mentioned leads to. The telescope of Sir William Herschel, to which 

 he applied a power of six thousand, did undoubtedly magnify the stars six 

 thousand times, but even then their apparent magnitude was inappreciable. 

 We are then to infer that the distance of these wonderful bodies is so enor- 

 mous compared with their actual magnitude, that their apparent diameter, seen 

 from our system, is above six thousand times less than any which the eye is 

 capable of perceiving. 



Under such circumstances it might appear hopeless to attempt to discover 

 the probable magnitude and brightness of the stars as compared with any stand- 

 ard known to us. Yet this problem, however hopeless it may seem, has 

 yielded to the ardor of astronomical inquiry. 



Dr. Wollaston instituted a series of observations and calculations, which 

 terminated in an estimate of the magnitude and brightness of the fixed stars as 

 compared with our sun. 



There are optical instruments called photometers, the nse and application of 

 which is to ascertain the comparative brightness of luminous objects. By 

 such instruments we can take any two visible luminous objects and compare 

 them so as to be enabled to say what is the numerical ratio of the lights which 

 they afford. Thus a common candle and a gas-lamp may be hied, and we 

 should be enabled immediately to say how many candles would be necessary 

 to give light equal to that of the lamp. 



By instruments of this species Dr. Wollaston prosecuted inves 

 object of which was to ascertain the numerical proportion between the light 

 afforded by the sun and that afforded by the stars. Let us take, for example, 

 the case of Sinus, or the dogstar. He found by such means, that tho Hgfat 

 received by us from Sirius was 20,000,000,000 of times less lhan that received 

 from the sun. This, be it observed, was a result not of theory or speculation, 



